In a conventional technology, when cracks are produced in surfaces of members constituting a nuclear reactor or any other similar structures, for example, the constitutional member is entirely exchanged or a reinforcing member is placed. In order to repair a cracked portion itself, various methods have been proposed, such as a method in which a cracked portion is removed by mechanical machining or electrical discharge machining and then filling the cracked portion with a suitable material by using TIG arc welding or laser welding and a method in which only a surface crack is welded and sealed.
In conventional surface crack sealing methods, the surface of a structural member on which a surface crack is produced is irradiated with a laser beam to be melted and focused through a lens, and a filler wire is supplied as a filler metal to the portion melted by the laser beam. The surface of the crack is thus melted and sealed.
In the conventional methods described above, only when water, oxides, or other substances are not present inside a defect, which is a surface crack, the surface crack is sealed by melting the cracked portion.
However, if water, oxides, and other substances are present inside the surface crack, in the sealing methods mentioned above, a mixture thereof is vaporized when heated by the laser beam. In this process, the melted metal in the laser repaired portion is blown off and pits are generated. The pits left behind may prevent the surface crack from being sealed.
In a conventional technology, in consideration of the above matter, there is provided a method in which a preceding laser beam and a subsequent laser beam are scanned along a common path to allow the preceding laser beam to heat or melt a surface defect site so as to remove water or contaminants in the surface crack and the subsequent laser beam to melt the opening of the surface defect so as to seal the surface defect (see Japanese Patent Application Laid-Open Publication No. 2003-320472 (Patent Document 1), for example).
In the conventional technique described above, since two laser beams are focused through a common optical system, it is difficult to individually or independently control the diameters of the preceding and subsequent laser beams, and hence, the preceding laser beam heats and melts the surface. If the surface is melted and the surface defect is sealed, the water and oxides left in the crack are vaporized by the heat in the subsequent sealing/welding step and the vapor pressure increases, which may produce pits in the sealing/welding step in some cases.